The internet Gopher protocol
a distributed document search and retrieval protocol
Bob Alberti, Farhad Anklesaria, Paul Lindner, Mark McCahill, Daniel Torrey
University of Minnesota Microcomputer and Workstation Networks Center
Spring 1991; Revised Spring 1992
gopher n. 1. Any of various short tailed, burrowing mammals of the family
Geomyidae, of North America. 2. (Amer. colloq.) Native or inhabitant of
Minnesota: the Gopher State. 3. (Amer. colloq.) One who runs errands, does
odd-jobs, fetches or delivers documents for office staff. 4. (computer tech.)
software following a simple protocol for burrowing through a TCP/IP internet.
Abstract
The internet Gopher protocol is designed for distributed document search and
retrieval. This document describes the protocol, lists some of the
implementations currently available, and has an overview of how to implement new
client and server applications.
The protocol and software follows a client-server model. Documents reside on
many autonomous servers on the Internet. Users run client software on their
desktop systems, connecting to a server and sending the server a selector (a
line of text, which may be empty) via a TCP connection at a well-known port.
The server responds with a block of text terminated by a period on a line by
itself and closes the connection. No state is retained by the server.
While documents (and services) reside on many servers, Gopher client software
presents users with a hierarchy of items and directories much like a file
system. The Gopher interface is designed to resemble a file system since a file
system is a good model for organizing documents and services; the user sees what
amounts to one big networked information system containing document items,
directory items, and full-text searching capabilities across subsets of the
information base.
Servers return either directory lists or documents. Each item in a directory is
identified by a type (the kind of object the item is), user-visible name (used
to browse and select from menu listings), an opaque selector string (typically
containing a pathname used by the destination host to locate the desired
object), a host name (which host to contact to obtain this item), and an IP port
number (the port at which the server process listens for connections.) The user
only sees the user-visible name. The client software can locate and retrieve
any item by the trio of selector, hostname, and port.
In submitting a query to a search server, the client sends the selector string
and the list of words to be matched. The response yields "virtual directory
listings" that contain files matching the search criteria.
Distribution of this document is unlimited. Please send comments to the Gopher
development team: <
[email protected]>. Experimentation with the
mechanisms described here is encouraged.
1. Introduction
The Internet Gopher protocol is designed primarily to act as a distributed
document delivery system. While documents (and services) reside on many
servers, Gopher client software presents users with a hierarchy of items and
directories much like a file system. In fact, the Gopher interface is designed
to resemble a file system since a file system is a good model for locating
documents and services. Why model a campus-wide information system after a file
system? Several reasons:
(a) A hierarchical arrangement of information is familiar to many users.
Hierarchical directories containing items (such as documents, servers, and
subdirectories) are widely used in electronic bulletin boards and other
campus-wide information systems. People who access a campus-wide information
server will expect some sort of hierarchical organization to the information
presented.
(b) A file-system style hierarchy can be expressed in a simple syntax. The
syntax used for the internet Gopher protocol is easily understandable, and was
designed to make debugging servers and clients easy. You can use Telnet to
simulate an internet Gopher client's requests and observe the responses from a
server. Special purpose software tools are not required. By keeping the syntax
of the pseudo-file system client/server protocol simple, we can also achieve
better performance for a very common user activity: browsing through the
directory hierarchy.
(c) Since Gopher originated in a University setting, one of the goals was for
departments to have the option of publishing information from their inexpensive
desktop machines, and since much of the information can be presented as simple
text files arranged in directories, a protocol modeled after afile system has
immediate utility. Because there can be a direct mapping from the file system on
the user's desktop machine to the directory structure published via the Gopher
protocol, the problem of writing server software for slow desktop systems is
minimized.
(d) A file system metaphor is extensible. By giving a "type" attribute to items
in the pseudo-file system, it is possible to accommodate documents other than
simple text documents. Complex database services can be handled as a separate
type of item. A file-system metaphor does not rule out search or database-style
queries for access to documents. A search-server type is also defined in this
pseudo-file system. Such servers return "virtual directories" or list of
documents matching user specified criteria.
2. The internet Gopher Model
A detailed BNF rendering of the internet Gopher syntax is available in the
appendix... but a close reading of the appendix may not be necessary to
understand the internet Gopher protocol.
In essence, the Gopher protocol consists of a client connecting to a server and
sending the server a selector (a line of text, which may be empty) via a TCP
connection. The server responds with a block of text terminated with a period
on a line by itself, and closes the connection. No state is retained by the
server between transactions with a client. The simple nature of the protocol
stems from the need to implement servers and clients for the slow, smaller
desktop computers (1 MB Macs and DOS machines), quickly, and efficiently.
Below is a simple example of a client/server interaction; more complex
interactions are dealt with later. Assume that a "well-known" Gopher server
(this may be duplicated, details are discussed later) listens at a well known
port for the campus (much like a domain-name server). The only configuration
information the client software retains is this server's name and port number
(in this example that machine is rawBits.micro.umn.edu and the port 70). In the
example below the # denotes the TAB character.
Client: (Opens connection to rawBits.micro.umn.edu at port 70)
Server: (Accepts connection but says nothing)
Client: <CR><LF> (Sends an empty line: Meaning "list what you have")
Server: (Sends a series of lines, each ending with CR LF)
0About internet Gopher#Stuff:About us#rawBits.micro.umn.edu#70
1Around the University of Minnesota#Z,5692,AUM#underdog.micro.umn.edu#70
1Microcomputer News & Prices#Prices/#pserver.bookstore.umn.edu#70
1Courses, Schedules, Calendars##events.ais.umn.edu#120
1Student-Staff Directories##uinfo.ais.umn.edu#70
1Departmental Publications#Stuff:DP:#rawBits.micro.umn.edu#70
(.....etc.....)
(Period on a line by itself)
(Server closes connection)
The first character on each line tells whether the line describes a document,
directory, CSO (qi) server, or error (characters '0', '1', '2', or '3'; there
are a handful more of these characters described later). The succeeding
characters up to the tab form a user display string to be shown to the user for
use in selecting this document (or directory) for retrieval. The first
character of the line is really defining the type of item described on this
line. In nearly every case, the Gopher client software will give the users some
sort of idea about what type of item this is (by displaying an icon, a short
text tag, or the like).
The characters following the tab, up to the next tab form a selector string that
the client software must send to the server to retrieve the document (or obtain
the directory listing of the directory). The selector string should mean
nothing to the client software; it should never be modified by the client. In
practice, the selector string is often a pathname or other file selector used by
the server to locate the item desired. The last two tab delimited fields denote
the domain-name of the host that has this document (or directory), and the port
at which to connect.
In the example, line 1 describes a document the user will see as "About internet
Gopher". To retrieve this document, the client software must send the retrieval
string: "Stuff:About us" to rawBits.micro.umn.edu at port 70. If the client
does this, the server will respond with the contents of the document, terminated
by a period on a line by itself. A client might present the user with a view of
the world something like the following window:
1. About internet Gopher
2. Around the University of Minnesota - Offices and Services/
3. Courses, Schedules, Calendars/
4. Events/
5. Microcomputer News & Prices/
6. Student-Staff Directories/
7. University Relations Information and Forms/
8. Weather for the Twin Cities
(A similar view is presented by the curses based UNIX gopher client.
The slashes on the ends of certain lines denote that the item is a directory.
The Mac client precedes the line with the icon of a file or a folder. )
The user does not know or care that the items up for selection may reside on
many different machines anywhere on the Internet.
Suppose the user selects the line "Microcomputer News & Prices". This appears
to be a directory, and so the user expects to see contents of the directory upon
request that it be fetched. The following lines illustrate the ensuing
client-server interaction:
Client: (Connects to pserver.bookstore.umn.edu at port 70)
Server: (Accepts connection but says nothing)
Client: Prices/ (Sends the magic string terminated by CRLF)
Server: (Sends a series of lines, each ending with CR LF)
0About Prices#Prices/Aboutus#pserver.bookstore.umn.edu#70
0Macintosh Prices#Prices/Mac#pserver.bookstore.umn.edu#70
0ZEOS Prices#Prices/ZEOS#pserver.bookstore.umn.edu#70
0IBM Prices#Prices/Ick#pserver.bookstore.umn.edu#70
0Printer & Peripheral Prices#Prices/PPP#pserver.bookstore.umn.edu#70
(.....etc.....)
(Period on a line by itself)
(Server closes connection)
3. More details
3.1 Locating services
Documents (or other services that may be viewed ultimately as documents, such as
a student-staff phonebook) are linked to the machine they are on by the trio of
selector string, machine domain-name, and IP port. It is anticipated that there
will be one well-known top-level or root server for an institution or campus.
The information on this server may be duplicated by one or more other servers to
avoid a single point of failure and to spread the load over several servers.
Departments that wish to put up their own departmental servers need to register
the machine name and port with the administrators of the top-level Gopher
server, much the same way as they register a machine name with the campus
domain-name server. An entry which points to the departmental server will then
be made at the top level server. This ensures that users will be able to
navigate their way down what amounts to a virtual hierarchical file system with
a well known root to any campus server if they desire.
Note that there is no requirement that a department register secondary servers
with the central top-level server; they may just place a link to the secondary
servers in their own primary servers. They may indeed place links to any
servers they desire in their own server, thus creating a customized view of
thethe Gopher information universe; links can of course point back at the
top-level server. The virtual (networked) file system is therefore an arbitrary
graph structure and not necessarily a rooted tree. The top-level (duplicated)
node is merely one convenient, well-known point of entry.
3.2 Server portability and naming
It is recommended that all registered servers have alias names that are used by
Gopher clients to locate them. Links to these servers should use these alias
names rather than the primary names. If information needs to be moved from one
machine to another, a simple change of domain name system alias names allows
this to occur without any reconfiguration of clients in the field. In short,
the domain name system may simply be used in the near term to re-map a server to
a new address. There is nothing to prevent secondary servers or services from
running on otherwise named servers or ports other than 70 , however these
should be reachable via a primary server.
3.3 Contacting server administrators
It is recommended that every server administrator have a document called "About
internet Gopher" as the first item in their server's top level directory. In
this document should be a short description of what the server holds, as well as
name, address, phone, and an e-mail address of the person who administers the
server. This provides a way for users to get immediate word to the
administrator of a server that is not running correctly. It is also recommended
that administrators place the date of last update in files for which such
information matters to the users.
3.4 Modular addition of services
The first character of each line in a server-supplied directory listing
indicates whether the item is a file (character '0'), a directory (character
'1'), or an error (character '3'). This is the base set of item types in the
Gopher protocol. It is desirable for clients to be able to use different
services and speak different protocols (simple ones such as finger; others such
as CSO (qi) phonebook service, or Telnet, or X.500 directory service) as needs
dictate. For example if a server-supplied directory listing marks a certain
item with type character '2', then it means that to use this item, the client
must speak the CSO (qi) protocol. This removes the need to be able to
anticipate all future needs and hard-wire them in the basic internet Gopher
protocol; it keeps the basic protocol extremely simple. In spite of this
simplicity, the scheme has the capability to expand and change with the times by
simply adding an agreed upon type-character for a new service. This also allows
the client implementations to evolve in a modular fashion, simply by dropping in
a module (or launching a new process) for some new service. The servers for the
new service of course have to know nothing about internet Gopher; they can just
be off-the shelf CSO, X.500, or other servers. We do not however, encourage
arbitrary or machine-specific proliferation of service types.
On the other hand, subsets of other document retrieval schemes may be mapped
onto the Gopher protocol by means of "gateway-servers". Examples of such
servers include Gopher-to-FTP gateways, Gopher-to-Archie gateways,
Gopher-to-WAIS gateways, etc. There are a number of advantages of such
mechanisms. First, a relatively powerful server machine inherits both the
intelligence and work, rather than the more modest, inexpensive desktop system
that typically runs client software. Clients do not have to be modified to take
advantage of a new resource.
3.5 Building clients
A client simply sends the retrieval string to a server if it wants to retrieve a
document or view the contents of a directory. Of course, each host may have
pointers to other hosts, resulting in a "graph" (not necessarily a rooted tree)
of hosts. The client software will save (or rather "stack") the locations that
it has visited in search of a document. The user will therefore always be able
to back out of the current location by unwinding the stack. If a client does
not understand what a say, type 'B' item (not a core item) is, then it simply
ignores the item in the directory listing; the user never even sees it. A
service (particularly a critical one) may be duplicated on more then one server.
A client unable to contact a particular server should try one of the duplicated
servers if they exist. Ideally, a client should pick one of the duplicated
servers at random to spread the load among servers.
3.6 Building ordinary internet Gopher servers
The retrieval string sent to the server might be a path to a file or directory.
It might be the name of a script, an application or even a query that generates
the document or directory returned. The server uses the string it gets up to
but not including a CR-LF or a TAB, whichever comes first. Following the
optional TAB is a date-time descriptor (YYYYMMDDhhmmss).
The TAB and date descriptor parts exist for the efficiency of full-text search
servers (description follows). If the TAB and date descriptor are present, the
server should return only items that have been modified since the specified
date-time descriptor. If the server cannot implement this By-Mod-Date
filtering, it can just discard the date descriptor. For example the selector:
Julius Caesar<CR><LF>
returns a directory listing if "Julius Caesar" is a directory selector, and
returns the file if "Julius Caesar" selects a file. The selector:
Julius Caesar<TAB>19910315000000<CR><LF>
if a directory selector, includes all sub-directories but only includes file
names that have been modified since 15 March 1991. If a file selector, the file
is only returned if it has been modified since 15 March 1991, otherwise an empty
return.
All intelligence is carried by the server implementation rather than the
protocol. What you build into more exotic servers is up to you. Server
implementations may grow as needs dictate and time allows.
3.7 Special purpose servers
There are two special server types (beyond the normal Gopher server) also
discussed below:
1. A server directory listing can point at a CSO (qi) nameserver (the server
returns a first character of '2') to allow a campus student-staff phonebook
lookup service. This may show up on the user's list of choices, perhaps
preceded by the icon of a phone-book. If this item is selected, the client
software will resort to a pure CSO nameserver protocol when it connects to the
appropriate host. We expect that client support for this to be superseded soon
by X.500 modules. The basic module of the client software would remain
unchanged; we would need to add an X.500 module.
2. A server can also point at a "full-text search server" (returns a first
character of '7'). To implement campus internet (or subnet) wide searching
capability, some machines may maintain full-text indexes on the contents of text
documents held by some subset of Gopher servers. A "full-text search server"
responds to client requests with a list of all documents that contain (or don't
contain) a one or more words. The client sends the server the selector string,
a tab, and the search string (words to search for). If the selector string is
empty, the client merely sends the search string. The server returns the
equivalent of a directory listing for documents matching the search criteria.
The words "and", "or", and "not" are reserved as Boolean operators, and
expressions with Boolean operators are evaluated from left to right. Example: a
client might specify the search criteria as "salmon and spinach or asparagus" to
a full-text search server and the server will respond in the normal Gopher
fashion, returning a flat list of documents that match the criteria.
The CSO addition exists for historical reasons: at time of design, the campus
phone-book servers at the University of Minnesota used the CSO protocol and it
seemed simplest to adapt to them. The index-server is however very much a
Gopher in spirit, albeit with a slight twist in the meaning of the
selector-string.
3.7.1 Building CSO-servers
A CSO Nameserver implementation for UNIX is available from Steve Dorner (anon
ftp from uxa.cso.uiuc.edu). We do not anticipate implementing it on other
machines.
3.7.2 Building full-text search servers
An full-text search server is a special-purpose server that knows about the
internet Gopher scheme for retrieving documents. These servers maintain a
full-text index of the contents of plain text documents on Gopher servers in
some specified domain. A gopher full-text search server was implemented using
several NeXTstations because we were able to take advantage of the full-text
index/search engine built into the NeXT system software. A search server for
generic UNIX systems based on the public domain WAIS search engine, is also
available.
By using several index servers (rather than a monolithic index server) we are
able to build and search indexes in parallel (although the client software is
not aware of this). While maintaining full-text indexes of documents distributed
over many machines may seem a daunting task, the task can be broken into smaller
pieces (update only a portion of the indexes, search several partial indexes in
parallel) so that it is manageable. By spreading this task over several small,
cheap (and fast) workstations we are able to take advantage of fine-grain
parallelism. Again, the client software is not aware of this. Client software
only needs to know that it can send a search string to an index server and
receives a list of documents that contain the words in the search string.
3.8 Item type characters
The client software decides what items are available by looking at the first
character of each line in a directory listing. Augmenting this list can
extend the protocol. A list of defined item-type characters follows:
0 Item is a file
1 Item is a directory
2 Item is a CSO (qi) phone-book server
3 Error
4 Item is a BinHexed Macintosh file. [Use of this type is discouraged]
5 Item is DOS binary archive of some sort. [Use of this type is discouraged]
6 Item is a UNIX uuencoded file. [Use of this type is discouraged]
7 Item is an Index-Search server.
8 Item points to a text-based telnet session.
9 Item is a binary file! Client must read until the connection closes. Beware.
+ Item is a redundant server (same information as the previous server)
Characters '0' through 'Z' are reserved. Local experiments should use other
characters. We discourage arbitrary, machine-specific extensions. Note that
for type 5 or type 9 the client must be prepared to read until the connection
closes. There will be no period at the end of the file; the contents of these
files are binary and the client must decide what to do with them based perhaps
on the .xxx extension. These binary types are experimental and largely
unsatisfactory. Some binary encoding scheme should really be used. Current
contenders are a simple headed block, uuencode or MIME base64 encoding... (Watch
this space!)
3.9 User display strings and server selector strings
User display strings are intended to be displayed on a line on a typical screen
for a user's viewing pleasure. While many screens can accommodate 80 character
lines, some space is needed to display a tag of some sort to tell the user what
sort of item this is. Because of this, the user display string should be kept
under 70 characters in length. Clients may truncate to a length convenient to
them. Selector strings sent to the server are most easily manipulated (both by
file system and server application) using short (255 byte) Pascal strings on
PCs... so selector strings should be less than 255 characters in length.
4 Simplicity is intentional
As far as possible we desire any new features to be carried as new protocols
that will be hidden behind new document-types. The internet Gopher philosophy
is:
(a) Intelligence is held by the server. Clients have the option of being able
to access new document types (different, other types of servers) by simply
recognizing the document-type character. Further intelligence to be borne by
the protocol should be minimized.
(b) The well-tempered server ought to send "text". Should this include tabs,
formfeeds, frufru? Probably not, but rude servers will probably send them
anyway. Publishers of documents will be given simple tools (filters) that will
alert them if there are any funny characters in the documents they wish to
publish, and give them the opportunity to strip the questionable characters out;
the publisher may well refuse. Note: Type 5 or 9 items are hacks for binary
file transmission that may change shortly. In these cases the server just sends
the binary and then closes the connection.
(c) The well-tempered client should do something reasonable with funny
characters received in text; filter them out, leave them in, whatever.
Appendix.
Paul's NQBNF (Not Quite BNF) for the Gopher Protocol.
Note: This is modified BNF (as used by the Pascal people) with a few
English modifiers thrown in. Stuff enclosed in '{}' can be
repeated zero or more times. Stuff in '[]' denotes a set of
items. The '-' operator denotes set subtraction.
Directory Entity
CR-LF ::= ASCII Carriage Return Character followed by Line Feed
character.
Tab ::= ASCII Tab character.
NUL ::= ASCII NUL character.
UNASCII ::= ASCII - [Tab CR-LF NUL].
Lastline ::= '.'CR-LF.
TextBlock ::= Block of ASCII text not containing Lastline pattern.
Type ::= UNASCII.
RedType ::= '+'.
User_Name ::= {UNASCII}.
Selector ::= {UNASCII}.
Host ::= {{UNASCII - ['.']} '.'} {UNASCII - ['.']}.
Note: This is a Fully Qualified Domain Name as defined in RFC 830.
(e.g. gopher.micro.umn.edu) Hosts that have a CR-LF
TAB or NUL in their name get what they deserve.
Digit ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' .
DigitSeq ::= digit {digit}.
Port ::= DigitSeq.
Note: Port corresponds the the TCP Port Number, its value should
be in the range [0..65535]; port 70 is officially assigned
to gopher.
DirEntity ::= Type User_Name Tab Selector Tab Host Tab Port CR-LF
{RedType User_Name Tab Selector Tab Host Tab
